1. Field of the Invention
The present invention generally relates to a method for manufacturing a zipper and, more particularly, to a method for manufacturing a watertight zipper that can effectively prevent the penetration of liquid.
2. Description of the Related Art
FIGS. 1 and 2 show a watertight zipper 9 manufactured according to a convention method. In such a method, a stringer tape 91 is coated with a resilient material to form a covering layer 92. Then, a plurality of apertures 93 is arranged on the stringer tape 91 and the covering layer 92 in even intervals. Finally, an injection molding is performed to bond a scoop 94 to the stringer tape 91 via a corresponding aperture 93. The scoop 94 includes an upper scoop portion and a lower scoop portion that are interconnected via a corresponding aperture 93. When the left and right scoops 94 are engaged with each other, the covering layers 92 of the left and right scoop portions may be firmly pressed against each other. As a result, the liquid is not able to penetrate the watertight zipper 9 from between the left and right scoop portions. Such a conventional manufacturing method can be seen in Taiwan Patent No. I339106.
In the conventional manufacturing method of the watertight zipper 9, a thermoplastic material is heated to a molten state, and the molten material is applied to the surfaces of the stringer tape 91 by extrusion. As such, the extruded molten material will form the covering layer 92 after cooling and solidification. However, the extrusion of the molten material will cause expansion of the thermoplastic material. Thus, the extruded molten material on the stringer tape 91 will have uneven expansion and shrinkage. As a result, the lateral edge 921 of the covering layer 92 is not even, such that the lateral edges 921 of the covering layer 92 cannot be in complete abutment without interstice when the left and right scoop portions are engaged with each other. Disadvantageously, liquid will slowly penetrate the watertight zipper 9 from between the left and right scoop portions.
Furthermore, the aperture 93 is in a circular form, and the upper and lower scoop portions of the scoop 94 are bonded together merely through the plastic material in the aperture 93. In this structure, if the scoop 94 is not properly combined with the molten material of the covering layer 92, the positioning of the scoop 94 will be disturbed by the pulling force operating the watertight zipper 9. As such, the operation of the watertight zipper 9 is not smooth.
Another type of the conventional watertight zipper can be seen in U.S. Pat. No. 2,910,754.
In light of this, it is necessary to improve the conventional manufacturing method of the watertight zipper.